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Sources and Magnitudes of Uncertainty in Fault Slip Rate, Cucamonga Thrust Fault, Southern California

Devin McPhillips, & Katherine M. Scharer

Published August 15, 2017, SCEC Contribution #7843, 2017 SCEC Annual Meeting Poster #122

Maps of historic earthquake ground surface ruptures show that displacements along strike are spatially variable. As a result, latest Quaternary slip rates developed from a small number of displacement measurements may not accurately represent fault motion. Moreover, displacement variability that persists over multiple ruptures may stem from fault processes rather than random noise. The Cucamonga thrust fault is part of a network of faults that have generated damaging historical earthquakes in the Los Angeles region. The Cucamonga fault is unusual in that numerous scarps along its 25 km length are well expressed on alluvial fans. In particular, the ~33 Qyf1 fan surfaces (Horner, 2006) are broadly preserved and feature all major fault strands. Using airborne lidar in combination with GPS surveys, we measured vertical separation (VS) at 46 locations, with multiple replicates at each location, in order to quantify along-strike variability. We also evaluated several potential sources of variance using morphometric tools. Results show considerable VS variance over a short length scale (~5 km). The observed variance systematically exceeds the expectation from measurement error alone by a factor of about 2.5, and locally by a factor of 5 or more. A limited correlation between VS and scarp-face gradient indicates that erosion is an important source of variance. However, the correlation does not hold on some of the faults strands and fans with the greatest observed variance, indicating that other sources are also significant. On the basis of stream profile inversions, we show that rock uplift rate in the hanging wall declines from east to west by ~30%, which we interpret as evidence of interactions between the Cucamonga thrust and nearby strike-slip faults. These interactions may allow for significant earthquake-to-earthquake rupture variability. Overall, our slip rate estimates are consistent with previous work on the Cucamonga fault, but our measurements demonstrate the considerable uncertainty in these values. The importance of scarp erosion to total uncertainty supports common practice of using maximum fault offsets for hazard assessments, but complex fault interactions, as well as other processes, may also contribute significantly to uncertainty in excess of measurement error.

McPhillips, D., & Scharer, K. M. (2017, 08). Sources and Magnitudes of Uncertainty in Fault Slip Rate, Cucamonga Thrust Fault, Southern California. Poster Presentation at 2017 SCEC Annual Meeting.

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